1. Field of the Invention
This invention relates to an evaporative fuel-adsorbing device for adsorbing evaporative fuel generated from a fuel tank of an internal combustion engine, and an evaporative emission control system for preventing evaporative fuel from being emitted into the atmosphere by the use of the evaporative fuel-adsorbing device.
2. Prior Art
Conventionally, an evaporative emission control system of this kind has been proposed by Japanese Provisional Patent Publication (Kokai) No. 1-159455. In addition to an ordinary canister for adsorbing evaporative fuel generated from a fuel tank when a vehicle in which the fuel tank is installed is parking or when an engine installed in the vehicle is operating, this evaporative emission control system includes a canister for exclusive use in refueling, which is adapted to adsorb evaporative fuel generated when the fuel tank is being refueled. Further, to solve the problem of a poor adsorbing efficiency of the ordinary canister due to a high flow velocity of evaporative fuel generated when the fuel tank is refueled, in the proposed evaporative emission control system, the canister for exclusive use in refueling is provided with partitions within the canister which divide its adsorbent-accommodating space into a plurality of layers, and the direction of flow of the evaporative fuel is changed to reduce the flow velocity of the evaporative fuel, thereby enhancing the adsorbing efficiency of the canister without increasing the size (ratio L/D (length/diameter)) of the canister.
However, with the conventional evaporative emission control system, the canister for refueling is intended exclusively for refueling, and it is not used on occasions other than at refueling. Once the evaporative fuel adsorbed is purged into the engine, it becomes empty, i.e. contains no evaporative fuel, resulting in a lower utilization factor of the activated carbon. Further, two batches of activated carbon have to be provided as adsorbents, one for the canister for exclusive use in refueling, and the other for the ordinary canister, which results in use of an increased amount of activated carbon, leading to an increased cost. Further, it is required to provide a plurality of purging passages connecting between the respective canisters and the intake passages for discharging evaporative fuel adsorbed in the canisters, which complicates the construction of the system.
Further, in the conventional evaporative emission control system, in purging a large amount of evaporative fuel for enhancing the utilization factor of the adsorbent, the flow resistance of a communication passage connecting between one layer of the adsorbent and another layer divided by a partition is large since the communication passage is narrow, which prevents a sufficient amount of evaporative fuel from being purged. As a result, it takes much time to purge all the evaporative fuel adsorbed by the adsorbent, and hence if traveling of the vehicle is repeatedly carried out at short time intervals, evaporative fuel adsorbed in the canister for refueling remains unpurged, resulting in a low utilization factor of the adsorbent.